1. (Field of Technology)
The present invention relates to a tracking system for an optical memory device of a type wherein any of the information recording, reproduction and erasing can be carried out by radiating a laser beam to a memory medium.
2. (Description of the Prior Art)
Optical memory devices have recently drawn the attention of people as a high density, high capacity memory device. The reason that an optical memory device is of high density and high capacity is because the size of each bit which represents a unit of information storage can be reduced to a diameter of about 1 .mu.m. This, in turn, however, imposes some limitations on the optical memory device. More specifically, in order for information to be recorded on, or reproduced from, a predetermined location, the light beam is required to be accurately positioned.
Because of the foregoing, when using a disc capable of recording information simultaneously with erasure of previously recorded information, it is a general practice for the disc substrate to be permanently provided with beam guide tracks or address information.
The guide tracks generally have a shape as shown in FIG. 10 of the accompanying drawings and are in the form of grooves of a depth generally equal to the wavelength .lambda. divided by the product of the refractive index n times 8, i.e., .lambda./8n. Any of the information recording, reproduction and erasing is carried out while the light beam scans the tracks of the disc guided along these guide grooves.
As a means for sensing a tracking signal from the guide grooves, two methods are well-known; a Twin Spot method (a three-beam method) such as generally used in association with VD (video disc) and CD (compact disc) and a push-pull method such as generally used in association with an optically writeable disc. The Twin Spot method and the push-pull method are illustrated respectively in FIGS. 12 and 13 of the accompanying drawings.
The Twin Spot method has an advantage in that a stable tracking performance can be achieved even though a pick up is inclined relative to the optical disc substrate. However, it has a problem in that, when a tracking beam scans a boundary between a guide groove region G and an address information region A constituted by a plurality of pits as shown in FIG. 12(a), the tracking tends to be disturbed because of the difference between a diffraction efficiency on the leading beam B.sub.1 and that on the trailing beam B.sub.2. It is to be noted that reference character R used in FIG. 12(b) represents recorded bits.
On the other hand, although the push-pull method is generally free from the above mentioned problem inherent in the Twin Spot method because of the tracking performed by a single beam B.sub.4 as shown in FIG. 13(a), it has a problem in that, because the position of the light beam which has been reflected towards a detector D shown in FIG. 13(b) tends to become displaced relative thereto in the event of a shift in position of a lens as a result of tracking or in the event of inclination of the pick-up relative to the disc, the tracking error signal tends to accompany a steady drift which will bring about a continuing shift in tracking. Accordingly, in the event that the pick-up has inclined relative to the disc, the pattern of diffraction occurring at the guide groove region and that at the address information region differ from each other and, as a result thereof, the amount of tracking shift necessarily deviates to such an extent as to result in a disturbed tracking at the boundary. FIGS. 12(a) to 12(c) and FIGS. 13(a) to 13(c) are schematic representations illustrative of the change in tracking error signal occurring during the tracking at the boundary according to these two methods, respectively.
In these figures, the servo region is considered to be sufficiently lower than the pit reproducing frequency and, therefore, an output of the detector during the tracking at the address information region is shown as an average value. FIG. 12 applies where the difference in amount of beam reflected is taken as a tracking error signal, whereas FIG. 13 applies where the difference in output from detectors for detecting two split beam components is taken as the tracking error signal.